This invention relates to efficient compounds and methods for dielectrically heating relatively non-conductive materials.
There are several benefits to an efficient process for recycling discarded plastics, such as polystyrene and styrofoam, into new, useful products. For example, consuming scrap plastic such as packing "peanuts" and styrofoam cups or dishes, rather than discarding the scrap plastic, has a beneficial impact on the environment by lessening the demand for landfill space. Additionally, recycling scrap may result in a lower demand for the raw materials and energy necessary to produce new plastic, once again benefiting the environment.
One known end use for recycled plastics is the creation of fire-retardant construction materials. For example, U.S. Pat. No. 4,596,682 discloses a phenolic resin plastic polymer for bonding shredded styrofoam chips into molded foam insulating panels and blocks for the construction industry. A significant feature of the polymer is that it produces a fire-retardant foam product, as opposed to high flammability typical of polystyrene and styrofoam.
Molded plastic items may be manufactured by creating a mold of the desired item introducing plastic resin into the mold, and heating the plastic resin until it begins to "cure" (heating the plastic until an exothermic reaction commences). One method of heating the plastic is to apply radiant heat. However, in practice, it was found that radiant heat sources simply cured or charred the outer surface of the plastic, but left the inside uncured. Thus, a different form of heating was required.
Another method of curing recycled plastics in a mold is to apply non-radiant heating. One form of non-radiant heating is microwave heating which heats primarily by agitating water molecules (resonant at 2.45 GHz) in the material to be heated. Microwave heating subjects poorly conductive materials to antenna-launched electromagnetic energy at microwave frequencies, typically at frequencies of about 2.45 GHz. U.S. Pat. No. 3,848,038 discloses an example of heating nonconductive materials with microwave energy. The thickness, size, and composition of material to be heated, however, limits the applicability of microwave heating.
Another form of non-radiant heating is dielectric heating. Similar to microwave heating, dielectric heating occurs when an electrically non-conductive material is subjected to radio-frequency ("RF") energy. However, dielectric heating is most commonly applied with an electric field, that is, between flat parallel plates of a capacitor, rather than an antenna, and at lower frequencies, typically 3-30 MHz, than microwave heating.
In dielectric heating, the electric field is generated by applied equal and opposite potentials on the two opposing metal plates of a capacitor. In a typical mold, the capacitor plates form the sides of the mold. A radio frequency power source ("RF source") is applied to the capacitor plates. Typically, one plate and one side of the RF source are at ground, and the other plate is connected to the high side of the RF source. The material to be molded is inserted between two plates of a capacitor. When the molding compound is a plastic resin, the dielectric heating will raise the resin to its polymerization point, triggering an exothermic reaction and hardening the resin.
Various types of plastic, such as recycled polystyrene foam, have such a high dielectric constant that minimal RF energy is absorbed, thereby prolonging the heating period. Long heating periods add recurring costs to the recycling process, and lower the process throughput.
In particular, expanded polystyrene beads have a very high dielectric constant, and absorb very little RF energy. Phenolic resins are primarily water by weight. Water readily absorbs microwave energy, but the loss factor for water decreases with decreasing frequency, and no longer readily absorbs RF energy at frequencies typical of available industrial radio RF sources. Additionally, microwave energy is not preferable for large (e.g., 48 inch by 96 inch) workpieces. The use of frequencies where the mold is a quarterwavelength or more in size causes uneven energy distribution and thus uneven heating. Also, large molds have resonant frequencies much lower than frequencies in the microwave range.
It is known to combine various additives with a dielectric to affect the RF energy absorbing characteristics of the dielectric. However, some prior attempts are not compatible with the water in phenolic based resins. See, for example, U.S. Pat. No. 4,790,965 (disclosing clay and earth additives, which should be dry to work) and U.S. Pat. No. 3,640,913 (disclosing zinc halide, which is anhydrous and incompatible with water-based resins). Other prior attempts operate at microwave frequencies, where a water based resin would readily absorb RF energy without any additive. See, for example, U.S. Pat. No. 3,848,038.
On aspect of the invention is to provide a novel dielectrically heatable compound, comprising a molding compound including, in one embodiment, plastic having a first dielectric constant and a water based resin; and a lossy additive, having a second dielectric constant substantially lower than the first dielectric constant. The plastic may comprise expanded polystyrene. The water based resin may comprise phenolic resin. The lossy additive may comprise graphite, carbon, or a mixture of graphite and carbon.
Another aspect of the invention is to provide a process for molding plastic by dielectrically heating the plastic in a mold with an RF source, comprising the steps of mixing plastic with a water-based resin, increasing a loss factor of the plastic and water-based resin, placing the plastic and water-based resin in the mold, generating RF energy with an RF source, and applying the RF energy to the mold. The step of generating RF energy with an RF source may, in one embodiment, include the step of generating RF energy in a range from 3 MHz to 30 MHz. The step of increasing the loss factor of the plastic and water-based resin may, in one embodiment, include the step of adding a low dielectric additive, such as carbon, or graphite, or both.